The present invention relates to digital diversity combining systems and, more particularly, to a digital hopped frequency, time diversity system.
Apparatus and techniques for radio communication between two points are well known in the prior art. While such communication is most commonly by way of modulating a voice signal on a carrier, techniques are also well known for modulating digital information in serial form on an appropriate carrier for radio transmission. The most common form of such transmission is a fixed point to point tranmission, with the locations of the transmitter and receiver and the transmitter power being selected so as to provide an adequate signal strength at the receiver under substantially all necessary atmospheric conditions.
In certain situations the transmitter or receiver may be mobile, such as by way of example, in police cars, public transportation vehicles, emergency vehicles and/or space, submarine and marine vehicles. Accordingly, signal reflections from buildings and the like and atmospheric conditions may temporarily cause the original signal and a reflected signal to arrive at the receiver with substantially the same amplitude but with opposite phasing, so as to result in serious signal fading and drop out.
There are two approaches which have been commonly used, either alone or together, to minimize the effects of these greatly varying signals. The first is to modulate the information to be transmitted, such as the voice signal, onto two carriers of differing frequencies and to transmit and receive both signals. Since the two signals have different wavelengths because of their different frequencies, it is unlikely that cancellation of both signals will occur simultaneously, so that the strongest signal or some combination of the two signals will provide a much more reliable signal than either signal individually. The second approach is to provide a plurality of receiving stations physically distributed around the expected area of travel of the transmitting station, again using either the strongest signal received or some combination of signals from all receivers. This space diversity, of course, also enhances the ability to derive a useful signal, as signal dropout at phically diverse locations at the same time is increasingly unlikely.
When using a multiplicity of space diversity, time diversity and frequency diversity, the likelihood is very high that at least one spacial-temporal signal is of reasonably high quality for one time increment or message block. The problem, however, is to select each acceptable short message block from the multitude of signals being received and then assemble the short acceptable blocks into the complete message. In the prior art in the case of voice communications, various techniques have been used to attempt to identify the best signal. Rapid and accurate identification of the best signal block is most difficult. In many cases the apparent best signal is manually selected by switching between channels to find one of the demodulated signals which is sufficiently intelligible for the intended communication purposes. However, this is rather tedious particularly under rapidly varying conditions, so that frequently a less than best signal is used until the extent of the fade-out is intollerable.
Certain automatic techniques have also been used for selecting the apparent "best" channel which represents a substantial improvement over the manually switched systems. These prior art systems generally depend upon some signal amplitude responsive measurement to either select the best apparent signal or to provide automatic gain control for each of the signals so that the combined signal emphasizes the better signals. However, since noise cannot be eliminate, such systems are typically responsive either to noise or signal plus noise rather than the quality or information content of the signal alone.
One prior art technique for improving the reception of an intelligible signal in a fading environment is to provide a non-coherent FSK (frequency shift keyed) signal at a rate of 16 Kb/s (kilobits per second) to send a block of 200 bits thirteen times on different hopped frequencies. The hop rate (chip rate) is assumed to be 80 hops per second. Before transmission, the bits are interleaved in a fashion such that two repeated bits are on two different frequencies. After reception, a 7-out-of-13 majority vote logic is used to determine each single message bit. This scheme provides an excellent performance but sacrifices a great deal of data throughput, for instance, 16 Kbs/13 approximately equals 1200 bits per second as compared with a data rate of 16 Kbs. The required transmission bandwidth for this repetition scheme is quite large. For a signal to noise ratio equal to 10 dB (decibel) and a BER (bit error rate) of 10.sup.-3, a repetition of 13 times is needed, that is, the required transmission bandwidth is 13 times the information rate.
As mentioned above, diversity combining techniques have been used for combating multipath fading in analog schemes, as well as digital schemes, in the past. In the range of very high frequency, the space diversity schemes are not feasible for mobile communications.